The present invention relates to an apparatus which is implanted in a human body to correct a deformed and/or degenerated spinal column, and more specifically relates to an apparatus which holds vertebrae in the spinal column in a desired spatial relationship.
A known apparatus for maintaining vertebrae in a desired spatial relationship in a spinal column is disclosed in U.S. Pat. No. 4,648,388, issued Mar. 10, 1987 and entitled "Apparatus and Method for Maintaining Vertebrae in a Desired Relationship". This known apparatus includes an elongate, cylindrical rod which extends along the spinal column. Connector assemblies are provided to connect the cylindrical rod to vertebrae in the spinal column. The rod maintains the vertebrae in a desired position.
Typically, the cylindrical rod has one diameter throughout its length. As a result, the surgeon may be faced with using a rod smaller in diameter than desired for a part of the spine. For example, the thickness of tissue between first vertebrae and the skin is less than the thickness of the tissue between second vertebrae and the skin. As a result, a rod of one diameter which is to extend between the first and second vertebrae will have to have a diameter which is small enough to be positioned between the first vertebrae and the skin along with the associated connectors and fasteners. However, such a rod may not have as large a diameter as may be desired to assist in carrying load. Therefore, the surgeon must compromise the load carrying capability of the rod because of the space requirements.